In conjunction with the storage of energy obtained in solar-thermal power stations, it is known for this energy to be stored in storage tanks with the aid of a molten salt as a heat carrier medium. This makes it possible to store energy in the heat carrier medium in the sunny phase during solar radiation, and to emit this energy in times when there is a lack of solar radiation, to the further heat transmission medium, which is flowing through the solar collectors, or the steam/water circuit of the solar power station, by pumping the heat carrier medium around, flowing through a heat transmission apparatus.
The heat transmission medium of the thermal energy store is stored, after this discharging process, in a further storage tank in the thermal energy store. For renewed charging, this heat transmission medium is then once again fed back into another or else into the previous storage tank, flowing through a heat transmission apparatus, in which the heat transmission medium circulating in the solar power station transmits heat or energy to the heat transmission medium of the thermal energy store during the sunshine phase.
Thermal energy stores of this type are known from practical use from a solar power station (SEGS I) which was commissioned in 1985. A thermal energy store such as this is also in use in the Solar Two trials power station in California. A thermal energy store in the form of a molten salt store has also been provided for the Andasol 1 solar power station that is currently being constructed, and this allows six hours of full-load operation of the Andasol 1 solar-thermal power station, without solar radiation. This thermal energy store comprises two storage tanks and has a total salt content of 28 000 t. In this case, as is illustrated in FIG. 1, the cooled salt is stored at a nominal temperature of 292° C. in a salt tank 1 which is “cold” after discharging, and the heated salt, or the heated molten salt, is stored after charging in the “hot” salt tank 2, at a nominal temperature of 384° C. At the start of the charging phase, the amount of salt or the molten salt is located in the salt tank 1 and is then pumped into the salt tank 2, as the second storage tank, during a sunshine and charging phase, by means of a feed apparatus 3, flowing through a heat transmission apparatus 4a, in which it is heated to the nominal temperature of about 384° C. by means of the further heat transmission medium, normally a thermal oil, which is circulating in the primary circuit of the solar-thermal power station. The molten salt is stored in the salt tank 2 until the discharge phase, which is illustrated in the right-hand part of the illustration in FIG. 1, starts during a phase when there is no sunshine. In this case, the molten salt is pumped back from the salt tank 2 into the salt tank 1, in the opposite direction to that during the charging phase, with the heat or energy which is stored in the molten salt or the heat transmission medium being emitted in the heat transmission apparatus 4b to the steam/water circuit of the power station, or possibly also to the further heat transmission medium which is flowing in the primary circuit of the solar-thermal power station.
The design of a thermal energy store such as this comprising a plurality of storage tanks depends on the solar radiation conditions at the respective site of the solar-thermal power station and on the energy demand that is satisfied by the power station. In this case, in particular, the energy demand which depends on the time of day and the time of year, and the energy provision strategy derived from this by the power station operator are particularly important.
Because of the installation-dependent constraints, the size of a storage tank is limited to about 15 000 m3, which means that the maximum amount of thermal energy which can be stored with one tank is 1200 MWh. In order to allow larger amounts of thermal energy to be stored, the previous concept consists of multiplying the number of tanks, that is to say in each case using two, four, six, eight etc. cold salt tanks 1 and two, four, six, eight etc. hot salt tanks 2, instead of in each case one cold salt tank 1 and one hot salt tank 2.
A multiple tank system such as this, which is known from practical use and comprises two cold salt tanks 1 and two hot salt tanks 2, is illustrated in FIG. 2.
In order to make it possible to provide an energy storage system having a tank size of more than 15 000 m3 and a storage capability of more than 1200 MWh, it has therefore been necessary until now to design a multiple tank system or a multiple tank configuration which is characterized by multiplication of the individual storage tanks on the so-called “cold” side (cold melt tank 1) and the “hot” side (hot salt tank 2). This leads to a considerable investment requirement which has to take account of each individual storage tank, and to a corresponding space requirement for each individual storage tank, and may have an adverse effect on the solar array which is provided for the solar-thermal power station by shadowing caused by the individual storage tanks.